Maybe you could test this like you would a direct mail package. Direct mail marketers will send out a letter or direct mail package to a sample list and test the response on that sample list. I am guessing you could do the same even though it is on the web.

Buy a mailing list or obtain a [permission based] email list and send out a test marketing piece to a sample of whichever or both lists and see what type of response rates you get and what type of conversion you get. If the response and conversion is high enough you roll it out to the entire list. If it is too low then adjust your copy or your list and test it again. Do this a few times and you will have a pretty good idea of what the best response and conversion rate will be.

In direct mail the common advice tossed around for years has been a response rate of 1-3% and this number became the "benchmark" which was really silly because some mailings produced 11-18% response and some even higher. It was simply getting the right combination of list, mail package, and offer in place to make it a success.

If I were doing this I would buy a direct mail list from a broker for the audience that your book would appeal to and send out a test mailing. Then I would research the web and find some e-Newsletters publishers that hit the same audience and contact the publisher and ask what kind of arrangement you could work out to either blast a direct response email to the list or include a direct response ad in one or more of the publications would be and then test it.

In direct marketing the old adage long before the Internet existed was test, test, and test. The formula did and still does work very well. This way you wouldn't have to guess or accept common "benchmarks" that may or may not have any true connection to your product.